The present invention relates to a method of constructing precast modular marine structures.
Much of the World""s production of oil and gas is derived from offshore wells. While the early offshore oil and gas fields were located in relatively shallow water, the need to develop oil fields in deep water has become more important as the shallow water oil and gas fields become depleted. As a result, many deep-water basins throughout the world have been opened to oil and gas exploration and drilling.
During the exploration for, and production of sub-sea resources like oil and gas, an array of marine vessels, structures and appurtenances are employed. Prior proposed vessels used for exploration, drilling, production and storage of oil and gas at sea included: ships, boats, mobile offshore drilling units, semi-submersible units, submersible units, jack-up rigs, platforms, spars, deep draft caisson vessels, tension leg platforms and various combination of these and other components often in conjunction with a riser or sub-sea system.
Platforms, spars, deep draft caisson vessels, and tension leg platforms typically include a long vertical cylindrical hull that supports a platform above the water line. The platform provides space for drilling and maintaining oil or gas wells where the production wells may be positioned along an outside edge of the platform. Alternatively, the production wells may be located in the center of the platform within a moon bay or pool. Likewise, the above water platform of such a marine structure can be configured for use such as a launch pad for aeronautical and space vehicles, housing, hotels, resorts, and manufacturing and processing facilities.
Generally, traditional construction methods and materials for marine structures, including platforms, spars, deep draft caisson vessels, tension leg platforms, jack-up rigs, semi-submersible units, mobile offshore drilling units, ships and boats require the erection of frames about which plates, planks or sheets of material such as metal, wood or resin impregnated cloth are faired by and attached (permanently or otherwise) to the frames by skilled labor to form a complete or at least a significant portion of the marine structure""s hull. Thereafter, the marine structure is launched or introduced into the water for further outfitting or operation.
Traditional materials of metal and/or wood require fairing, fixing and supporting the material(s) between frames. However, due to limitations in the structural and strength characteristics of traditional construction materials and the lack of economical labor with the proper skills, alternative construction methods have been developed. For example, the world""s first metal oil/gas production spar hull was constructed as two separate sections in Finland. The two separate sections were shipped across the Atlantic Ocean aboard heavy lift vessels until reaching the Gulf of Mexico. There, the two separate sections of the spar hull were brought back to shore and welded together. The entire welded hull was then towed horizontally to the project site and upended to the vertical position by filling its lower ballast tanks with water.
Marine structures, such as the Troll A Platform, have been constructed from concrete materials using the slip form construction technique. This technique typically calls for the pouring of concrete in a vertically movable form. The form is connected to jack rods with hydraulic jacks, which move the form vertically in minute increments as the concrete is being poured. Once pouring begins, it continues until the top of the structure is reached, allowing for a monolithic poured concrete structure. Utilizing the slip form construction technique for marine structures requires a transportation path of sufficient clearances (in terms of water depth and overhead clearances) to accommodate the vertical monolithic poured structure. Furthermore, the scantlings of the lower regions of the pour must be of sufficient strength to accommodate the weight of the upper regions of the structure while being poured.
The structural sections may include either plated hull tank sections, or a combination of tank and truss-type section. An example of such spar platforms is depicted in U.S. Pat. No. 5,558,467 issued on Sep. 24, 1996 to Horton (hereinafter Horton ""467). The Horton ""467 patent describes a hull having a passage longitudinally extending through the hull in which risers run down to the sea floor. However, the Horton ""467 patent fails to provide for a precast modular marine structure or incorporation of an equalized pressure system that adjusts internal pressure of the structure in relation to external pressure, namely hydrostatic pressure, exerted thereupon.
An alternative design of an existing spar platform is depicted in U.S. Pat. No. 5,875,728 issued on Mar. 2, 1999 to Ayers, et al. (hereinafter Ayers ""728). The Ayers ""728 patent provides for a spar platform incorporating an essentially vertical cylindrical buoyant vessel and a shroud surrounding the vessel. The shroud includes two intersecting sets of foam-filled fiberglass elements that are secured to the vessel using standoffs. Nevertheless, the Ayers ""728 patent neither describes nor claims a precast modular marine structure or incorporation of an equalized pressure system, which gives the structure the ability to withstand an increasing hydrostatic force as the water depth increases.
Without an equalized pressure system, a spar system and any other marine structure requires additional reinforcement to withstand the significant hydrostatic forces. Such structures, including spars, risers, tension legs, and buoyancy cans must include greater wall thickness; stronger, lightweight materials; pressure resistant shapes; pre-pressurization of the structure and combinations of these techniques, especially when operating water depth increases. Utilizing the greatest wall thickness to withstand the maximum hydrostatic pressure over the complete depth of operation of the marine structure results in a simplified construction, but with a significant increase in weight and limit upon the ultimate water depth at which the marine structure can operate. A significant weight reduction can be achieved by varying the wall thickness in relation to the depth of water. Such a solution, however, significantly increases the complexity and cost to construct the marine structure, yielding only a modest increase in the limit of the ultimate operating water depth. The same result is true with the use of stronger lightweight materials, different shapes or combinations of the same. Each of these approaches use the strength of the construction material to withstand the hydrostatic pressure exerted on the external surface or wall of a typically hollow, closed marine structure.
Another known solution requires an increase in the internal pressure of the marine structure to a pressure that approximates the hydrostatic pressure that will be experienced at the depth at which the structure is planned to be operated. The obvious goal is to significantly reduce or eliminate the pressure differential experienced at the marine structure""s wall. One approach is to pre-pressurize the marine structure, or compartments thereof, in order to eliminate or significantly reduce the pressure differential that will be experienced once the marine structure is located in its operational position. As can be appreciated, pre-pressurization calls for designing the marine structure to be, in effect, a pressure vessel with a positive pressure contained inside until finally positioned at the prescribed depth. This pre-pressurization requires increased wall thickness and presents a potential safety hazard because of the often-high pressures that must be contained within the vessel during handling prior to, and during installation. One method of delaying pre-pressurization is contemplated in U.S. Pat. No. 5,636,943 issued on Jun. 10, 1997 to Haney (hereinafter Haney ""943). According to Haney ""943, gas is automatically generated on the inside of the tubular member as the structure descends to its optimal location. However, gas generation is dependent upon the consumption of pre-installed chemicals and a one-time reaction involving such chemicals.
In view of the above-described complexities associated with the design and use of known marine structures, which by their nature were usually designed and constructed to withstand significant internal-external pressure differentials across an outer wall or hull, the present invention has been developed to alleviate these drawbacks and provide further benefits to the user. These enhancements and benefits are described in greater detail herein below with respect to several alternative embodiments of the present invention.
The present invention in its several disclosed embodiments alleviates the drawbacks described above with respect to conventionally designed and constructed marine structures and incorporates several additionally beneficial features further enhancing the design and construction of such structures. Specifically, the present invention contemplates a novel method of constructing a precast, modular spar system for drilling, oil and gas production, and oil storage in a variety of water depths. The spar incorporates arcuate-shaped concrete segments cast and assembled onshore to form a cylindrical module having a central longitudinal passageway. The modules are assembled onshore to form cylindrical units which are then assembled onshore or offshore to form the final cylindrical spar of the desired length and width for the specific production site. In the event the final assembly of the spar occurs onshore, the structure is towed horizontally to the production site and upended. If the final assembly of the spar occurs offshore, the modules are towed either vertically or horizontally to the production site. At the production site, the modules are vertically assembled to form the final spar structure. The spar is adapted to have a length in which its normal draft places the bottom of the spar at a location sufficiently below the water surface that the effect of waves is attenuated to very low amplitudes and wave excitation forces are relatively small. The heave motion of the spar may thereby be reduced to almost zero even in the most severe seas while surge, sway, roll and pitch motions remain within readily acceptable limits.
The invention further contemplates an equalized pressure system including a vertical column of water with a segmental length positioned concentrically along the entire length of the buoyant section of the spar and an equalized pressure pipe system for pressurizing the interior compartments of the segments to equal the pressure of the adjacent sea water. The equalized pressure pipe system is also used in the upending process and in maintaining a constant draft of the spar at the specific production site.
The present invention is intended to provide:
(a) a spar of novel precast modular construction which can be economically used from shallow to deep water applications for oil storage facilities, oil and gas production facilities, and a riser system;
(b) an independent structure which can be used with several different types of production systems;
(c) a structure which has low sensitivity to fatigue or sea water corrosion, and which is resistant to the chemical and mechanical deterioration associated with freezing and thawing;
(d) a spar buoy which provides enhanced stability in a floating catenary moored condition;
(e) a novel, inexpensive precast modular construction method for structures used from shallow to deep water applications; and
(f) a novel equalized pressure system equalizing a hydrostatic pressure differential experienced at a wall of a marine structure at a predetermined operational water depth.
As an independent structure, the present invention may take the form of a spar which can be used with several different types of production systems such as tension leg platforms, semi-submersible platforms, FPSO""s or to support topside production, facilities and crew living structure. As can be appreciated, the enhanced stability of a marine structure with at least one spar lends itself to supporting an oil/gas production package, hotel accommodations, launch pad, runway, heliport or other activities which require a stable payload platform. A further purpose of the invention is to provide a simple, inexpensively constructed modular marine structure, such as a spar, with an equalized pressure system capable of equalizing a hydrostatic pressure differential experienced at a wall of the marine structure at a predetermined operational water depth.
The novel precast modular construction method simplifies the required structural engineering by the repetitive use of rings or pre-cast modular units. The pre-cast modular units are cast and erected on land to form the substantial portion or the whole marine structure. Construction of the structure with pre-tensioned and post-tensioned reinforced concrete provides an extremely large safety fatigue factor. The standard construction aids in fabrication plant productivity and quality control. Structural engineering is simplified and uniform wall thicknesses can be achieved because a novel equalizing pressure system is utilized to equalize the pressure differential across the submerged portion of the marine structure""s hull or wall.
In its simplest form, the equalizing pressure system includes a pressurized gas source fluidly connected via a conduit to at least two internal compartments of a marine structure (like a spar system) designed to be located underwater for at least portions of the structure""s operation life. The compartments are fluidly connected to each other to allow gas and water to flow between the compartments and the water column, which substantially surrounds the marine structure.
As may be appreciated, if an interior compartment of a marine structure is open at its bottom to the surrounding water column, the pressure differential across the marine structure""s hull plating adjacent to the interior compartment will be equal to, or nearly zero regardless of the depth at which the compartment is located. Furthermore, by positioning a fluid passage at a lower portion of the compartment, gas can be pumped through the passage and into the compartment to be trapped in an upper portion thereof. As the gas pressure increases in the fluid passage, water exits through the bottom opening of the compartment. If the gas pressure in the fluid passage decreases, water moves into the compartment through the bottom opening, and any gas in the compartment is compressed to a pressure substantially equal to the hydrostatic pressure at the bottom opening. In this manner, the pressure within the compartment is substantially equal to the hydrostatic pressure at the bottom opening. If the marine structure has a significant height, there will be a pressure differential gradient experienced along the height of the hull plating or wall since the interior pressure will be uniformly equal to the hydrostatic pressure at the bottom opening while the hydrostatic pressure on the outside of the marine structure will vary with respect to depth. Normally, a particular marine structure will have a height sufficiently short where this gradient presents little effect. If, however, the marine structure is significantly tall, it may be easily segmented into a plurality of one-above-the-other compartments, each having an individualized equalizing capability. By controlling the balance between the volume of water and gas in the compartment, the buoyant effects experienced upon the marine structure can be altered.
In another aspect, the equalizing pressure system of the present invention further includes a pressurized gas source fluidly connected via a conduit system to two or more compartments of a marine structure situated in water. Each compartment has a passage configured to allow gas and/or water to freely pass between the lower region of a compartment and the water, which surrounds the marine structure. The conduit system has a manifold positioned between the gas source and a plurality of pipes, each of which connects to the two or more compartments. The conduit system permits selective and variable control of the buoyancy factor obtainable from the vessel.
In a further embodiment, the gas source is fluidly connected via a segmented conduit system to two or more compartments of a marine structure situated in water. The segmented conduit system is configured to allow gas and/or water to flow between adjacent compartments and the body of water in which the marine structure is situated.
While the invention is described as an equalizing pressure system for marine structures, it is clearly possible to apply the same system and methods to other structures, fluids and/or materials where pressure equalization is desired between interior and exterior spaces of a vessel; and it is permissible that at least a limited amount of exterior surrounding fluid, whether it be liquid or gas, migrate between the two spaces.
The beneficial effects described above apply generally to the exemplary devices and mechanisms disclosed herein for an equalizing pressure vessel typified as an underwater buoyancy vessel. The specific structures through which these benefits are delivered will be described in detail herein below.